The present invention relates to an apparatus for feeding a fluid medium and, more particularly, to an apparatus for feeding a fluid medium acting as an electrolyte, especially a fuel.
An apparatus of this kind for feeding a fluid medium is known comprising a housing having an inlet opening and an outlet opening for the fluid medium, an electric drive motor located in the housing and having an armature spindle, and a pump also mounted in the housing in working connection with the drive motor. The electric drive motor has a commutator device connectable electrically to an electric circuit. The commutator device comprises a commutator having contacting surfaces mounted on the armature spindle of the electric drive motor, at least two contacting brushes engaged with the contacting surfaces of the commutator and prestressed spring means associated with each of the contacting brushes, each of the contacting brushes being held against the contacting surfaces by action of the prestressed spring means.
So that the known device (as described in German Published Patent Application 30 16 086) can be used for feeding of a fluid medium acting as an electrolyte, the individual wires of the copper conductor cable leading from the electrical connection points to the carbon brushes located in a case- or box-like guide are surrounded by a protective coating of nickel, tin, silver or aluminum. This is because in the alcohol fuel under consideration for use, which is more powerful in comparison to current fuels and which contains for example 15% methanol (M 15), the electrical conductivity is in the Micro-Siemens-Region (.mu.S). This is based on the fact that methanol has an affinity for water. In pure methanol (M 100) or ethanol (E 100) fuel the electrical conductivity increases because of its susceptibility to oxidation and the formation of acetic acid (in E 100) and formic acid (in M 100). In the Micro-Siemens-Region of the electrical conductivity these fluids already act as electrolytes, so that because of the potential differences in the motor deposits occur, especially on the bare wires on the anode side which are exposed to fluid medium flow.
With the usually used fuel, whose conductivity is in the Pico-Siemens-Region (pS), these considerations play no part in the design of the fuel feed apparatus. The features of the feed apparatus known from the above mentioned state of the art for prevention of electroerosion stiffen the brush wires so much that the usual well made contact of the commutator brush to the contacting surface of the commutator is no longer guaranteed with the usual brush spring means formed as a compressed coil spring. A corresponding stiffening of the spring however increases the wear on the carbon brushes and decreases the efficiency of the electric drive motor and also the feed apparatus. Additional basic problems are described in the German Published Patent Application 30 16 086.